This invention relates to magnetrons.
Magnetrons typically include (FIG. 1) a main body portion 1, an anode 2, often with vanes 3 to define resonant cavities 4, a coaxial cathode 5, means (not shown) for setting up a magnetic field parallel to the axis of the cathode, and an antenna 6 coupled to a probe 7 in a resonant cavity 4, for launching r.f. energy into a waveguide 8. The space between the anode and the cathode is evacuated, and the antenna 6 is also within the evacuated region, a glass dome 9 connected to a copper sleeve 10 forming part of the envelope.
The thickness of the glass dome 9 is not great, so the effect on the electrical length of the output is not great and, while the window does cause a mismatch because the r.f. energy encounters a change of dielectric constant resulting in reflections, the effect of this is reduced by the dome shape.
However, the manufacturing operation required to seal the glass dome to the copper sleeve 10 is time-consuming (a so-called Housekeeper copper/glass seal has to be formed due to the expansion coefficient of the glass) and therefore expensive.